I. Field of the Invention
The present invention relates to a method of treating steel sheets, whereby the steel sheet is electrolytically treated in a suspension solution, and then is cured by heat treatment.
The above-mentioned suspension solution consists of at least one water-dispersible sol of a metal compound, a water-soluble or water-dispersible organic polymer and an infusible powder of a polymer resin. Furthermore, the suspension solution can contain at least one powder of a metal, an alloy of said metal and a difficultly water-soluble or water-insoluble compound of the said metal.
It is an object of the present invention to produce the steel sheet coated by composite materials consisting of an organic polymer, a metal powder and/or metal compound, said product having excellent corrosion resistance because of the uniform coating with composite materials.
II. Description of the Prior Art
Conventional, well-known methods of producing a coated metal sheet, by use of a mainly polymer resin, generally, are shown as follows:
1. By the method of roller coating, knife coating, spray coating and immersion coating, the organic solvent-type paint is coated on the metal sheet and then is cured by heat treatment.
2. By the above-mentioned coating method, the mixture of polymer resin powder and adhesive solution is coated on the steel sheet, and then is cured by heat treatment.
3. By the method of injection coating, the polymer resin powder is coated on the metal sheet.
4. By the method of electrophoresis coating, the metal sheet is coated with the organic solvent containing the polymer resin powder, dried and then is cured by heat treatment.
5. By the method of electrophoresis coating, the metal sheet is coated with a treatment solution containing a water-dispersible polymer resin and then is cured by heat treatment.
6. By the method of electrostatic coating, the polymer resin powder is coated on the metal sheet, and then is cured by heat treatment.
In the above-mentioned methods of coating (1), (2) and (3), it is difficult to control the thickness or uniformity of the coating. Uniformity of the coating thickness with regard to quality control is often one of the most troubling factors. Also, for the application of a thin coating thickness, the said methods are very disadvantageous from an economical standpoint. Further, when the powders of a polymer resin and inorganic compound are used as coating materials, these methods become more and more difficult.
In the case of methods (1), (2) and (4), wherein an organic solvent is used, we often find that a public nuisance is caused by the use of such solvent.
The method of (5) requires that a strong ionic charge be produced by dissociation of the polymer resin. Therefore, the polymer resin used must be carefully chosen. For example, insoluble nylon, polyethylene and others cannot be electrodeposited by the method of (5).
The method of (6) has a weak point in that the adhesion of the resin powder is poor and the coated powder is often easily separated from the base metal sheet by a little impact or contact after production. In cases where the coating contains metal powder, the coating adhesion deteriorates further.
On the other hand, according to Japanese patent publication No. Sho 50-75623, a metal sheet is cathodically electrotreated in a suspension solution consisting of alumina sol of 1 to 500 m.mu. (particle diameter) with a positive electric charge and an organic polymer resin. By use of this method, one can easily control the coating thickness by the amount of electricity on treating. It is not necessary to use harmful organic solvents, therefore, a public nuisance is not caused by the treatment solution. Also, the above-mentioned insoluble nylon, polyethylene, etc. and inorganic powder can easily be electrodeposited in the suspension solution as described above.